1. Field of the Invention
The present invention relates to seismic imaging, and more particularly to seismic imaging with tube-wave excitation and detection.
2. Description of the Relevant Art
U.S. Pat. No. 6,591,193, entitled “Method and apparatus for acquiring offset checkshot survey data using tube-wave conversion”, incorporated herein by reference, discloses a method for acquiring offset checkshot survey data for the subsurface region in the vicinity of a fluid-filled well, said method comprising the steps of: deploying an acoustic receiver at a known depth in said well; determining the tube-wave traveltime from said acoustic receiver to a tube-wave conversion point located in said well; generating a seismic signal at a source location that is laterally offset from said well; measuring the total traveltime of said seismic signal along a ray path from said source location to said tube-wave conversion point and then through said fluid to said acoustic receiver; and subtracting said tube-wave traveltime from said total traveltime to determine the seismic signal traveltime from said source location.
Tube-waves have traditionally been regarded as a source of high amplitude noise in borehole seismic data, and great effort typically goes into their suppression and elimination from recordings. Tube-waves have very large amplitudes and can propagate long distances without substantial decay. A tube-wave is an interface wave for a cylindrical interface between two media, typically a borehole fluid and surrounding elastic rock. Borehole waves were described by Lamb and were observed in the early twentieth century, as summarized by White. Using trapped (or guided) mode analysis, the classic tube-wave can be seen as the lowest order trapped mode. Higher order modes may be generated depending on wave propagation material properties and source frequency. The fundamental mode is usually referred to as a Stoneley wave. Some work has been done to analyze tube-wave attributes in order to evaluate rock properties. The conversion of tube-waves into a coal seam trapped modes was reported by Albright and Johnson.